Method for insuring high silicon carbide content in elnisil coatings

ABSTRACT

A highly wear-resistant nickel-silicon carbide composite is applied to a cast metal surface by a process in which the cast metal surface is cathodically subjected to high current density in an aqueous acid treatment bath, preferably containing nickel sulfamate in a concentration of at least 600 grams per liter. A dispersion of silicon carbide particles is maintained in the bath in a concentration of 90 to 150 grams per liter. The bath temperature is regulated to be at least 150*F and cathodic current density is maintained at a level of at least 200 a.s.f. The anode is shaped to conform with the internal configuration of the cathode surface to be coated; significant bath agitation by air is employed to maintain the silicon carbide particles in continuous suspension to insure such particles settle out along with the electrodeposited nickel in a predetermined uniform and highly dense distribution. The process is continued until a coating thickness of at least 25 mils is deposited within a time period of no greater than 180 minutes.

United States Patent [1 Cordone et al.

[ METHOD FOR INSURING HIGH SILICON CARBIDE CONTENT iN ELNISIL COATINGS[75] Inventors: Leonard G. Cordone, Allen Park;

William A. Donakowski; John R. Morgan, both of Dearbom Heights; KarlRoemming, Detroit, all of Mich.

[73] Assignee: Ford Motor Company, Dearborn, Mich.

[51] Int. Cl. C25D /02; CD 3/12 [58] Field of Search 204/3, 4, 16, 49

[56] References Cited UNITED STATES PATENTS 3,06l,525 lO/l962 Grazen..204/3 3,l32,927 5/l964 Borner 204/9 3,326,782 6/1967 Kendrick et al.204/3 3,640,799 2/1972 v Stephan 204/38 B [451 June 24, 1975 PrimaryExaminer-T. M. Tufariello Attorney, Ager'lt, 0r Firm.loseph W. Malleck;Keith L. Zerschling 5 7 1 ABSTRACT A highly wear-resistantnickel-silicon carbide composite is applied to a cast metal surface by aprocess in which the cast metal surface is cathodically subjected tohigh current density in an aqueous acid treatment bath, preferablycontaining nickel sulfamate in a concentration of at least 600 grams perliter. A dispersion of silicon carbide particles is maintained in thebath in a concentration of to grams per liter. The bath temperature isregulated to be at least l50F and cathodic current density is maintainedat a level of at least 200 a.s.f. The anode is shaped to conform withthe internal configuration of the cathode surface to be coated;significant bath agitation by air is employed to maintain the siliconcarbide particles in continuous suspension to insure such particlessettle out along with the electrodeposited nickel in a predetermineduniform and highly dense distribution. The process is continued until acoating thickness of at least 25 mils is deposited within a time periodof no greater than minutes.

5 Claims, 2 Drawing Figures [412 Ava/72727107? METHOD FOR INSURING HIGHSILICON CARBIDE CONTENT IN ELNISIL COATINGS BACKGROUND OF THE INVENTIONThe process of the present invention is particularly applicable to themanufacture of composite articles requiring a functional coating, suchas in a rotor housing for rotary internal combustion engine, with theexposed coating ofthe composite housing capable of serving ad mirablyunder severe wear conditions.

Most commonly employed wear-resistant materials are heterogeneous instructure, that is, they contain two or more distinct, chemicallydifferent phases since, in most cases, pure homogeneous metals havenotoriously poor wear characteristics. Since wear is entirely a surfacephenomenon, engineers for many years have employed techniques to form adispersion at the surface of parts subject to wear. Such techniquesinclude operations such as nitriding, case hardening, hard facing flame,metalizing and alike. In these surface treatments, metallurgical lawscontinue to place severe restrictions on the physics and the chemistryof the coatings produced.

The process of the present invention is concerned with a new approach tothe preparation of dispersions. A dispersion coating is deposited hereinby electrolytic means in such thicknesses to constitute a functionalpart, but the coating must be deposited within a reasonably short periodof time so that the cost of fabrication is not exorbitant. Asimultaneous electrodeposition of a metal and settling out ofelectrically inert additive particles is promoted under controlledconditions to produce a composite dispersion structure in which theadditive particles retain their discrete identity and are homogeneouslydispersed within the electroplated metal deposit. The selection of thechemistry of the particular additive particles will, of course, begoverned by the characteristics desired in the final product. Where theprimary concern is the improvement of wear-resistance, material such assilicon carbide, aluminum oxide, tungsten carbide, titanium carbides,Zirconium oxide, boron carbide, chromium carbide, or diamonds may beemployed. I

It has been found most difficult to deposit a functionally thick coatingonto a rotor housing of an internal combustion engine which is shaped asan epitrochoid and wherein the additive particles are in sufficientquantity and distribution to promote the characteristics of wear that isexpected of the rotor housing. Since the wear characteristic is highlydependent upon the density and distribution of the silicon carbide inertparticles, it has been extremely difficult to obtain satisfactoryresults while attempting to increase the speed of plating to make such afunctional coating economically feasible and commercially practical.

SUMMARY OF THE INVENTION A primary object of this invention is toprovide a process capable of electrolytically depositing a heavy thickfunctional dispersion coating more quickly and more efficiently than theprior art.

Still another object of this invention is to provide a method forelectrodepositing a matrix of nickel and a dispersion of silicon carbideparticles, the carbide particles being distributed with a high degree ofhomogeniety and uniform density to promote a high wear resistance for acoating useful as a wear surface in a rotary internal combustion engine.

Yet another object is to provide a method for electrodepositing acomposite of nickel and silicon carbide onto a cathodic article to becoated, the process utilizing a semiconforming anode shapedpredominantly to the configuration of the surface to be coated, thespacing between the anode and cathode article normally inhibiting auniform deposited dispersion but compensated for by the method.

SUMMARY OF THE DRAWINGS FIG. 1 is a photomicrograph of a portion of arotor housing for a rotary internal combustion engine having a chemistrysimilar to the deposited coating of the invention but havingunsatisfactory wear characteristics;

FIG. 2 is a photomicrograph of a portion of a rotor housing having afunctionally thick coating of nickel and silicon carbide according tothe teaching of this invention and having a highly satisfactorywear-resistant characteristic.

DETAILED SPECIFICATION 1 electrolytically deposited from a bathcontaining nickel sulfamate in a uniquely high concentration and siliconcarbide particles in a unique agitated concentration, can beelectrolyzed by a high current density to deposit out a relatively thickcoating having an unprecedented uniform, dense'and homogeneousdistribution of silicon carbide particles rendering unusually high wearresistance. It is now known that the bath may contain any nickel saltselected from the group consisting of nickel sulfate, nickel chloride,nickel fluoroborate, nickel sulfamate and any mixtures thereof. Inaddition, the particles may be any inert particle havingthe requiredhardness shape and temperature stability.

APPARATUS The problem of uniformly distributing carbide particles in anickel matrix is particularly evident when attempting toelectrolytically deposit a coating on the epitrochoid surface formingthe internal wall of the rotor housing of a rotary internal combustionengine.

A preferred method mode for carrying out the present inventionessentially comprises:

a. providing an aqueous acidic solution containing nickel sulfamate in aconcentration of at least 600 grams/liter, nickel chloride in aconcentration of about 17 grams/liter, boric acid (H in a concentrationof about 45 grams/liter, saccharin (acting as a stress reliever) in aconcentration of about 2.2 grams/liter, and silicon carbide particles ina concentration of about grams/liter, the latter having a fine particlesize of about 3 microns and substantially spherically shaped.

b. continuously introducing air into said solution at a plurality ofstations disposed uniformly throughout a bottom horizontal zone of saidsolution, the flow rate of said air being controlled to be in the rangeof 75-l25 c.c./minute/cm to agitate the ingredients of said solution andprevent laminar conditions or phase variation therein.

c. applying an electrical potential between a cathodically disposedarticle and a nickel containing anode assembly in said solution toprovide a current density at said article of at least 200 amps/ft andpreferably at 400 amps/ft during a predominant later stage of saiddeposition.

d. continuing said deposition until a composite coating of at least 25mils is obtained in a time period no greater than l80 minutes.

A typical apparatus mode for carrying out the present invention maycomprise a solution tank in which is disposed a series'of stacked rotorhousings which have been previously cast from aluminum metal. The rotorhousings have an internal epitrochoid surface and flat side walls sothat it is convenient to separate them by electrically inert materialsuch as plastic sheets and to protect the exposed top and bottom wallsof the stack by means of aluminum plates coated with silicone rubber toeliminate deposition thereon. The stacked housings may be carried on aharness for movement into i and out of the solution. The housings areeach connected by suitable means to an electrical potential so housingsin a predetermined spaced relation no greater than 0.75-l.0 inches.Masking bands may be mounted about the sleeve'to block current throw atintervals between housings or around edges to promote a uniform coatingthickness on the cathode epitrochoid surface. The anode assembly may beconstructed as a titanium mesh basket wherein the sleeve is constitutedof expanded sheet metal or-woven wire; nickel pieces are ar- 1 ranged inintimate contact with each other within the basket. The titanium meshbasket is placed in contact with a suitable voltage source tooperate asa current transfer agent, being anodically inert due to the developmentof an oxide coating. The nickel piecesin contact with the mesh basketthus become the anode while the basket serves as a constant anode area.

BATH MAKEUP The bath makeup should principally and preferably becomprised of an aqueous solution of nickel sulfamate Ni (NH- S0 having aconcentration of at least 600 grams per liter, a concentration ofsilicon carbide particles of 100-150 grams/liter.

Satisfactory nickel sulfamate compositions are available commercially,for example, as M&T sulfamate from Metals and Thermite of Matawan, N.J.,or as Bar- I rett sulfamate from Allied Research Company of Baltimore,Md. Any commercially available nickel sulfamate which is controlled inthe amounts of contaminate present by prior substantial removal of ironand similar metals can be satisfactorily used in the sulfamate platingbath of this invention.

The sulfamate plating bath should also contain angrams/liter, preferably17 grams/liter. An ionizable metal salt, such as one or more of sodiumchloride. sodium fluoride, sodium bromide, potassium chloride, potassiumbromide. and the like, (Ni (31 -611 0) can be used to supply thenecessary ingredient. Nickel chloride is a particularly preferredionizable metal salt for use in the sulfamate bath of the presentprocess. The function of the ionizable salt is to promote smoothdissolution of the consumable, nickel anode pieces. If the anode piecesare not reacted, the current density of the anode cannot be increasedbeyond 200 amps/ft without causing the detrimental evolution of gas.'lnorder to plate an article at the cathode current densities required ofthis invention, the surface area of the anode would normally have to beincreased two or vthree times beyond'the'surfaee area of conventionalanode assemblies. It will be readily appreciated that this is difficultto arrange where substantially conforming anodes are needed. Nickelchloride further increases the stress in the deposits formed.Nonetheless, the concentration of the nickel chloride should be kept aslow as practicable because coatings formed at the highest currentdensities are under considerable stress already and the increase in thestress is not of such great importance.

Boric acid (H 80 should also be present in the sulfamate plating bath ofthis invention in a concentration of between 37-45 grams/liter, andpreferably about 45 grams/liter. Boric acid acts as a buffer in theplating solution and ifpresent in the required amount, is effective inproviding a smooth, uniform and non-graining plated surface. l

A sufficient amount of a wetting agent'is added'tothe sulfamate platingsolution to regulate the surface tension of thebath between about-2O anddymes/centimeter Baths having a surface tension in this range have beenfound to be more amenable to proper agita- .tion duringelectroplatingand therefor allow; the production of a 'more uniform composite. Aparticularly helpful wetting agent is saccharin or sodium lauryl sulfatein the range of 1.7-2.7

V grams/liter or preferably about 2.5 grams/liter.

The pH of the sulfamate plating bath must be mainacid; boric acid, ornickelcarbonate may be added to i the plating bath, as needed tomaintain the proper operating range. V

Nickel sulfamate plating solutions provided in accordance with thisinvention have certain additional advantages over known platingsolutions. n particular, the throwing power of the electrolyte issuperior to that s of other solutions. A method for measuring throwing 1other ionizable metal salt in a concentration of l5l9f powerutilizes ahull cell described in the paper entitled if a dominant nickel sulfamatebath isnot used. the r V nickel salt should be selected from the groupconsisting of nickel sulfate, nickel chloride, nickel fluoroborate,nickel sulfamate, and any mixtures thereof.

It is important to avoid metallic powders in these baths, for example,poorly cast niekel'anodes which might powder during the use anddispersed nickel particles in the bath definitely can cause roughnessand also high concentrations of activated carbon bath cause veryundesirable roughness. Carbon from rolled or cast nickel anodes,however, does not usually cause roughness when floating in the baths.High concentrations of iron dissolved as ferrous or ferric iron in thebaths do cause settling roughness or gross pitting effects in the bathsat pH values of 3.8 to 5.5. Zinc or cadmium ions can be present in thebaths in concentration as high as about 0.02 grams per liter withoutdetrimentally effecting the plating. The presence of sodium andmagnesium salts are not harmful.

The selection of the particular additive particles will, of course, begoverned by the characteristics desired in the final product. Where theprimary concern is the improvement of wear-resistance, and abrasionresistance,

and the like, materials such as silicon carbide, aluminum oxide,tungsten carbide, titanium carbide, zirconium oxide, boron carbide,chromium carbide, thorium oxide or uranium oxide, may be employed. Theshape should preferably be acicular or spherical and of a particle sizeno greater than 400 mesh or 3 microns. The concentration of the additiveparticles must be in the range of lOO-l50 grams/liter.

The deposition of the additive particles under the surface of thecathodic particle to be coated does not require any cataphoretic action,but it is due solely to the settling of the particles through the bodyof the liquid and on to the article surface. Since the voltage employedin this and conventional electroplating operations is usually ontheorder of 12 volts or less, the voltage gradients present in theelectrolyte are insufficient to cause any cataphoretic deposition of theadditive particles.

CURRENT DENSITY It has been found that as the concentration of thenickel salt, nickel sulfamate, is varied, the maximum current densityand the internal stress in the deposit both vary. As the concentrationof nickel sulfamate is varied from 550 up to 900 grams per liter, themaximum current density that may be applied rises substantially to asmuch as 400 amps/ft in the range of 600-700 grams per liter andthen'falls again.

The higher the cathode current density the greater is the speed ofdeposition. But under given plating conditions, there is a practicaimaximum current density above which so-called burning occurs. The valueof this practical maximum current density depends not only on thetemperature of operation but also on the rate of agitation and thedeposition of the anode and the cathode in the plating bath. A typicalmaximum current density for the prior art is about 200 amps/ft for aplating solution containing about 300 grams/liter of nickel sulfamate ata temperature of about 60C. in accordance with this invention a maximumcurrent density is about 400 amps/ft for a plating solution containingabout 600-700 grams/liter of nickel sulfamate at a temperature of about160F. Initially the plating sequence may use only about 200 amps/ft butthe predominant portion of the plating process can be carried out atorclose to the maximum current density.

The temperature of operation has a material affect on the maximumcurrent density as well as upon the internal stress. If the temperatureis reduced from 65C to 50C, the internal stress moves rapidly in thetensile direction. Below 50C, tensile stress in the deposit is so highand the maximum practical current density is so low that the use of thesolution offers no significant advantage over conventional solutions.However, as the solution is heated to higher temperatures, the maximumcurrent density is accordingly raised, with a compressive stress whichis higher but the tensile stress being lower. This phenomenon can beemployed in electrodeposition or electroforming since the electroformproduct should have a low internal tensile stress if it is to have adimension stability required for many purposes such as a uniform coatingon the inside of an epitrochoid surface. The required bath temperatureherein is between 7l-77C or preferably about 160F.

PRODUCT The resulting product of practicing the above method willproduce a dispersion coating having certain unique features. A highsilicon carbide content in the range of 35% by weight will be assured.The distribution, density and character of the dispersed siliconparticles will be of a highly desirable type as exhibited in thephotomicrograph of FIG. 2. This is in contrast to the randomness andheterogeneity of the silicon carbide particles when processed accordingto the prior art, as shown in FIG. 2. For this latter exhibit, the bathconcentration was about 320 grams/liter of nickel sulfamate, siliconcarbide in a concentration of about grams/liter, no independent bathagitation, and a cathode current density below 200 amps/ft in someembodiments, a substantially pure nickel strike may be deposited on thecast aluminum substrate in advance of depositing the dispersion coatingthereover, such as shown in FIG. 2.

Tests were performed to compare engine wearresistance of compositecoating herein as a function of the percentage weight or volume ofsilicon carbide in the end product. When the bath contained between86-97 grams/liter of silicon carbide, the end product had an averagecarbide weight of 3.3% and an average volume content of 9.5%. Afterhours of engine test work, considerable chatter (grooves in the coating)and gouling occurred with the electroplated coating. When the bathcontained between 100-112 grams/- liter, the average carbide weight was4.3% and the average volume 11%. Engine wear tests showed such a coatingto have good wear resistance. When the bath contained about -150grams/liter, the average carbide weight was in excess of 4.7% and theaverage volume was in excess of 12.1%. At the end of 100 hours of enginetesting, there was perceivable wear and no measurable wear was observedat 400 hours of testing. Thus, the latter concentration must beconsidered superior.

We claim as our invention:

1. A method for electrolytically depositing a highly uniformwear-resistant dispersion coating onto a cast substrate, comprising:

a. providing an aqueous acidic solution containing at least one nickelsalt selected from the group consisting of nickel sulfate, nickelchloride, nickel fluoroborate, and nickel sulfamate and any mixturesthereof, said salt being present in a concentration of at least 600grams/liter, said solution containing also a suspension of inertwear-resistant particles in a concentration of about l50 grams/liter anda prec. applying an electrical potential between a nickel 7 containinganode assembly and said cast substrate to provide a current density atsaid cast substrate of about 400 amps/ft? 2. The method of claim 1, inwhich said aqueous solution being particularly comprised of at least 600grams/liter of nickel sulfamate, about l7 grams/liter of nickelchloride, about 45 grams/liter of boric acid, and about 150 grams/litersilicon carbide, said substrate is defined as an epitrochoid' surfaceand said anode assembly is substantially conforming: in configuration tosaid surface with spacing therebetween being extending up to 1.0 inch asa result of the increased throwing powerof said solution at said highcurrent densities.

3. The method of claim I, in which said step (c) is carried out for aperiod of time no greater than 180' minutes with a deposited dispersionno less than 25 mils thick. g

4. The resulting product of practicing the method of claim 1 in whichthe content of inert particles in said dispersion coating is homogeneousand yet in the range of 35%' by weight and the volume in the 'range'of5. The resulting product as in claim 4, in which said dispersion coatingis effective toJexperience no greater than 0.0005 inches of wear over aperiod of 100 hours i of engine performance when said substrate is therotor housing of a rotary internal combustion engine.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 5 Patent No. 3 891542 Dated June 24 197 Leonard G. Cordone et al. Page 1 of 3 Inventor s)It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Add Figures 1 and 2, as part of Letters Patent 3,891,542,

as shown on the attached sheets.

Signed and Scaled this Fifth Day of Apn'l1977 [SEAL] Attesi:

RUTH C. MASON C. MARSHALL DANN Anesn'ng Officer Commissioner oflarentsand Trademarks Patent No. 3,891,542 Page 2 Page 3 Patent No. 3,891,542

A/Zm/J/Ww

1. A METHOD FOR ELECTROLYTICALLY DEPOSITING A HIGHLY UNIFORMWEAR-RESISTANT DISPERSION COATING ONTO A CAST SUBSTRATE, COMPRISING: A.PROVIDING AN AQUEOUS ACIDIC SOLUTION CONTAINING AT LEAST ONE NICKEL SALTSELECTED FROM THE GROUP CONSISTING OF NICKEL SULFATE, NICKEL CHLORIDE,NICKEL FLUOROBORATE, AND NICKEL SULFAMATE AND ANY MIXTURES THEREOF, SAIDSALT BEING PRESENT IN A CONCENTRATION OF AT LEAST 600 GRAMS/LITER, SAIDSOLUTION CONTAINING ALSO A SUSPERSION OF INERT WEAR-RESISTANT PARTICLESIN A CONCENTRATION OF ABOUT 150 GRAMS/LITER AND A PREDOMINANT PARTICLESIZE ON GREATER THAN 3 MICRONS AND HAVING A SHAPE WHICH IS SPHERICAL ORACICULAR SO AS TO DESPERSE QUICKLELY THROUGH SAID SOLUTION, SAIDSOLUTION BEING MAINTAINED AT A TEMPERATURE NO LESS THAN 160*F, B.INTRODUCING AN INERT GASEOUS MEDIUM INTO SAID SOLUTION AT A PLURALITY OFSTATIONS DISPOSED UNIFORMLY THROUGHOUT A BOTTOM HORIZONTAL ZONE OF SAIDSOLUTION AND AT A FLOW RATE OF BETWEEN 75-125 C.C./MINUTE/CM2 TOTURBULATE SAID SOOLUTION IN A MANNER FOR PREVENTING LAMINAR CONDITONS INSAID SOLUTION, AND C. APPLYING AN ELECTRICAL POTENTIAL BETWEEN A NICKELCONTAINING ANODE ASSEMBLY AND SAID CAST SUBSTRATE TO PROVIDE A CURRENTDENSITY AT SAID CAST SUBSTRATE OF ABOUT 400 AMPS/FT2.
 2. The method ofclaim 1, in which said aqueous solution being particularly comprised ofat least 600 grams/liter of nickel sulfamate, about 17 grams/liter ofnickel chloride, about 45 grams/liter of boric acid, and about 150grams/liter silicon carbide, said substrate is defined as an epitrochoidsurface and said anode assembly is substantially conforming inconfiguration to said surface with spacing therebetween being extendingup to 1.0 inch as a result of the increased throwing power of saidsolution at said high current densities.
 3. The method of claim 1, inwhich said step (c) is carried out for a period of time no greater than180 minutes with a deposited dispersion no less than 25 mils thick. 4.The resulting product of practicing the method of claim 1 in which thecOntent of inert particles in said dispersion coating is homogeneous andyet in the range of 3-5% by weight and the volume in the range of10-15%.
 5. The resulting product as in claim 4, in which said dispersioncoating is effective to experience no greater than 0.0005 inches of wearover a period of 100 hours of engine performance when said substrate isthe rotor housing of a rotary internal combustion engine.